1. Field of the Invention
The present invention relates to an organic electroluminescent element and a display device using the same.
2. Description of the Related Art
Organic luminescent elements employing organic materials are expected to be promising in applications such as inexpensive, full-color, wide flat-panel display devices of the solid luminescent type and writing light source arrays, and thus development of the elements has been carried out extensively. An organic luminescent element in general consists of a luminescent layer, and a pair of opposing electrodes between which the luminescent layer is disposed. When an electric field is applied between the two electrodes, electrons are injected from the cathode and holes are injected from the anode. Luminescence is a phenomenon in which energy is emitted in the form of light, when the electrons and the holes are recombined in the luminescent layer, and the electrons return to the valence band from the conductive band.
Conventional organic luminescent elements require high driving voltage and exhibits low luminescence luminance and luminescence efficiency. In recent years, however, various techniques for solving these problems have been reported.
Japanese Patent Publication Laid-Open (JP-A) 2003-68466 discloses that it is possible, by means of a luminescent element having an anode and a cathode formed on a substrate, and an organic luminescent layer disposed between the anode and the cathode, wherein the organic luminescent layer consists of a host material and a dopant incorporated in this host material, and the dopant comprises a light-emitting material and a non-light-emitting compound, to achieve operation at a low voltage and to obtain high luminance, high efficiency and high durability.
However, in the system utilizing BCP (Bathocuproine) described therein, since BCP has a high ionization potential (6.1 eV), satisfactory durability cannot be obtained, and improvement is desired.
JP-A 2003-77674 discloses an electroluminescent element having a luminescent layer which comprises (1) a host material having an electron transport or hole transport property, (2) Compound A exhibiting phosphorescence emission at room temperature, and (3) Compound B exhibiting phosphorescence emission or fluorescence emission at room temperature and having a maximum light emission wavelength longer than a maximum light emission wavelength of Compound A, wherein Compound B is thereby made capable of light emission with a high efficiency. That is, it has been discovered that with respect to Compound B which is a phosphorescent compound not emitting light with high efficiency by itself, or which is a fluorescent compound emitting light in various colors but incapable of ensuring light emission efficiency as high as that of the phosphorescent compound for any of those colors, when Compound A of the above constituent (2) which exhibits phosphorescence emission at room temperature is used in combination with the foregoing Compound B, Compound A plays a role of sensitizer, and the light emission of Compound B is intensified. However, the luminescent element described in the JP-A 2003-77674 requires a high driving voltage.
Accordingly, there is a need for an organic electroluminescent element which exhibits high external quantum efficiency, superior durability, and low driving voltage.